I use my little finger (baby, pinky) on the RH when it's more convenient than trying to avoid it. But it doesnb't work well for trills and mordants and other ornaments, and in the upper reaches of the keyboard the pinky just won't reach.
Regards fingering I favour "flexible fingering"; and will state catagoricly that there is no "correct fingering" for the Hayden system concertina! My little fingers I use very flexibly on the RHS for the leading notes, and for chromatic decorations, and usually find that I am using ti IV (up) do I, re II, fa III, which fits the RH very comfortably, but might also use do II, & re III, then mi IV, up to Fa I, So II, La III.
I wondered if anyone dared use the pinky on the LH side. I almost never use the LH pinky, even tho it would make bass runs/riffs/scales smoother. Partly due to its inability to land on the right button with any degree of success, and partly because I have to leave a long fingernail on it, due to a chronic sinus condiiton
On the LHS I use my little finger almost entirely for Bass notes around an octave below my other fingers which may be playing counter melody or chords. I have trained it to go right across even under Left I.
At least the pinky has no problems pushing the nice easy light buttons on the Bastari 67. --Mike K.